1. Field of the Invention
The present invention relates to an ignition control system for a passive safety device and, more particularly, to an ignition control system suitable for ignition control of a passive safety device that includes a head protecting airbag device.
2. Description of the Related Art
A passive safety device including a head protecting airbag is described in, for example, WO 96/26087. In such a passive safety device, the head protecting airbag is housed extending along a front pillar and a roof side rail of a vehicle. If a side impact on the vehicle or a roll-over of the vehicle is detected, the head protecting airbag is deployed into a space between the head of an occupant and a side portion of the vehicle. Thus, the head protecting airbag appropriately protects an occupant's head at the time of occurrence of a side impact or a roll-over.
For effective operation of the above-described passive safety device, it is necessary or effective to provide a roll-over sensor that precisely detects a roll-over of a vehicle and to provide a seatbelt pretensioner that operates, upon detection of a roll-over, to restrain an occupant from moving sideways relative to the vehicle so as to secure a space between a vehicle side portion and the occupant into which the head protecting airbag is deployed. The head protecting airbag is operated to protect an occupant's head not only at the time of a roll-over but also at the time of a side impact. Therefore, in some passive safety device ignition control systems, a roll-over sensor is omitted, and the head protecting airbag is operated only when a side impact is detected.
That is, the passive safety device ignition control systems can be divided into two types: basic systems which detect the occurrence of frontal impacts and the occurrence of side impacts and, on the basis of the information detected regarding impact, control the operation of a frontal impact airbag, a side impact airbag, a head protecting airbag and a seatbelt pretensioner (hereinafter, the functions will be referred to as "basic functions"); expanded systems which, in addition to the basic functions, detect the occurrence of a roll-over and, on the basis of the information detected regarding roll-over, control the head protecting airbag and the seatbelt pretensioner (hereinafter, the functions will be referred to as "roll-over functions").
Since the head protecting airbag and the seatbelt pretensioner need to operate at the time of a side impact and at the time of a roll-over as described above, the ignition circuit provided in a basic system has the function of igniting the head protecting airbag and the seatbelt pretensioner. Therefore, expanded systems are analogous to a basic system to which a roll-over detecting sensor has been added and which, and at the time of detection of a roll-over, employs the ignition circuit of the basic system to ignite the head protecting airbag and the seatbelt pretensioner.
However, if such an expanded system with the roll-over detecting function is adopted, the electrical conditions of the ignition circuit, such as the impedance thereof and the like, change from those of the ignition circuit in a system without the roll-over detecting function. Therefore, it becomes necessary to adjust the circuit constants or factors of the ignition circuit, and the like, in accordance with whether the ignition circuit is used by the basic system or the expanded system. As a result, there arises the danger of causing drawbacks, such as a cost increase of the ignition control system and the like.